Noise clipping circuitry for television receivers



June 21, 1960 A. MACOVSKI 2,942,062

NOISE CLIPPING CIRCUITRY FOR TELEVISON RECEIVERS Filed June 1, 1955 2 Sheets-Sheet 1 I Z0 W050 0 CUZU/F 0P 5- W 1 V/DE W050 W 5 mvpz, 05/1100. HMPZ. rem/5mm:

INVENTOR.

June 21, 1960 A. MACOVSKI 2,942,062

NOISE CLIPPING CIRCUITRY FOR TELEVISON RECEIVERS Filed June 1, 1955 2 Sheets-Sheet 2 United States Patent NOISE CLIPPING CIRCUITRY FOR TELEVISION RECEIVERS Albert Macovski, Massapequa, N.Y., assignor to Radio Corporation of America, a corporation of Delaware Filed June 1, 1955, Ser. No. 512,476

5 Claims. (Cl. 178-73) The invention relates to synchronizing pulse separating and automatic gain control voltage generating cirouitry for television receivers, and it particularly pertains to such circuitry incorporating means to clip any noise that may appear beyond the tips of the synchronizing pulses of the received composite television signal. In present television receiver design practice noise pulse voltages existing in the extra-black region are nullified at the image reproducing device or kinescope by designing the input circuit to the kinescope with the cut-off voltage level at the black level of the video signal wave. The composite video wave, however, is also applied to the automatic gain control (A.G.C.) voltage generating tube and to the synchronizing pulse separating circuitry where such noise pulse voltages can cause poor synchronization, resulting in vertical rolling or horizontal tearing of the reproduced image. Although this noise is usually 'in the form of short duration voltage peaks, it is sufiicient to charge the integrating capacitor in the vertical synchronizing pulse integrating circuit, causing it to reach the triggering level of the vertical deflection oscillator earlier than it should. In the horizontal synchronizing circuit these noise pulses can be mistaken for horizontal synchronizing pulses themselves. To reduce the effect of this noise on the synchronizing circuitry, short timeconstant circuits are included in the synchronizing circuitry and'the video frequency (V.F.) amplifying circuitry is designed so that the tips of the synchronizing pulses are at the cut-off level of the video amplifying tube to clip the noise pulse voltages that extend beyond the tips of the synchronizing pulses. This effectively dictates the use of a sharp cut-off type V.-F. amplifier tube and also limits the gain that can be obtained with any tube chosen for this service considerably below the gain which the tube is capable of delivering when it is operated in the most favorable portion of its characteristic.

An, object of the invention is to provide noise immunity for the synchronizing circuitry of a television receiver and at the same time obtain greater gain from the video frequency amplifier tube than has been possible with the prior art arrangements.

Another object of the invention is to provide a circuit arrangement for clipping noise voltages that extend above the tips of the synchronizing pulses of a video frequency signal at the input tube the synchronizing pulse separating circuitry and the automatic gain control voltage circuitry without requiring the additional parts and components.

According to the invention the composite video frequency wave is applied to the grid of an A.G.C. voltage generating electron discharge tube which is arranged with the cathode at the same direct potential as the tips of the synchronizing pulses of the video frequency wave so that noise pulse voltages extending beyond the tips are clipped by the diode action of the grid-to-cathode circuit of the A.G.C. tube and the input circuit of the synchro- 2 nizing pulse separator is connected. to the grid of the A.G.C. tube so that the noise clipped wave is also applied to synchronizing circuitry. The A.G.C., action holds the tips of the synchronizing pulses close to the zero bias level of the A.G.C. tube to assure good noise clippingat all times.

In order that the practical aspects of the invention may be more fully appreciated and the invention readily put to use, an express embodiment thereof, given by way of example only, will be described with reference to the accompanying drawing in which:

Fig. 1 is a functional diagram of portions of a television receiver to which the circuitry according to the invention may be applied;

Fig. 2 is a schematicdiagra-m of an example of circuitry performing the functions outlined in Fig. l and incorporating circuitry according to the invention; and

Fig. 3 is a graphical representation of waveformsuseful in explanation of the operation of circuitry according to the invention.

A functional diagram of a television receiver to which noise clipping circuitry according to the invention is readily applicable is shown in Fig. 1. Such a television re ceiver will otherwise comprise circuits which may be entirely conventional and which will be described to illustrate the setting of the invention. In such a receiver television signals appearing at an antenna are applied to a radio frequency wave amplifying circuit and the out putt herefrom is applied along with the wave obtained from a local beat oscillation generating circuit to a frequency changing circuit. The output of the frequency changing circuit is applied at input terminals 15 to a picture intermediate frequency (I.F.) amplifying circuit 16. The LP. amplifying circuit 16 may be an individual picture LF. amplifying circuit or one amplifying both picture and sound signals. The demodulating circuit 17 is coupled to the LP. amplifying circuit 16 for deriving a video wave from the television signals. Detected video signals are amplified in a radio frequency (RF) amplifying circuit 18 and thereafter applied by way of the signal translating circuit 19 to the input circuit of an image reproducing device, or kinescope 20. The translating circuit 19 may consist of brightness and color difference signal translating channels of a color television receiver circuit 16, or from the demodulating circuit 17, or from the video amplifying circuit 18 in accordance with one aspect of the invention for further processing in a sound signal channel comprising an aural signal discriminating circuit, an audio frequency amplifying circuit, and a transducer, the latter usually being in the form of a loud speaker. The output of the video amplifying circuit 18 is also applied to circuitry 24 of which the synchronizing pulses are separated from the picture information and the horizontal synchronizing pulses are separated from the vertical deflection wave generating circuit and horizontal synchronizing pulses are app-lied to a horizontal oscillator and frequency control circuit 27. The vertical deflection wave is amplified in a vertical wave amplifying circuit and the horizontal deflection wave is amplified in a horizontal deflection wave amplifying circuit 28 to which a high voltage generating circuit is connected, and in the case of a color television receiver fo-,

" para e "or the videoa'r'nplifyiii c transforrn'er conr e'cte' frt iei1e a p the IF. amplifying energizing potentials to all circuits.

aegaoeg ri t. e tg rta g' mfents pt this type either ei f mm l z fig .cir-

cuitry is 1:

noise pulsesextending 'beyondft he'tips. .Of the Synchroniz- 'in'g' pulses! Usually this is do e; by holding" the yideo am i yi bewhigh ,clips mo stpfoflth'e, noise fpiilses Y but also 'ope'ratesthe V.1F., amplifying tubefinthe loyvg regionjof theeharacteristic 'cu'rveat 'whi'chthe gain is low 'and at which egiomt-he 'characteris'ticf'i far from l:ine ar.j For the flatter "feaso'n 's'h'arp'fc utroif ,pent'ode tube is'usually used Whereas a 'remot'e'clit? tube would provide better resultsr It i ae rable tliat the'VQF. tnbe he operatfid d eirj a'subs' ntialIyIiheafpQ tion of the characteristic curve' so'that'the "415 \megae' ae fre'qu'eney modulated sound v'vav'etrnay eampnfied beforeapplication 'to'fhe' sound channeh V v Accordin'g 'jtojthe invention fthe'j'contr ol grid;cat hode ci1fuif bf'aiifA.G,C, tubers, liZed toclipth'e fnoise pulses'fextending beyond, the tips of, the synchronizing and the' input -circiiit of {the Synchronizing, separaea te' the A.G.C.. tube "grid so "that" the jvva'y'e is 'a'pplied 'tofthe synchro- I eircui t The A'. GLC voltage a efpicft'ufe Ij'F 'a'rnfplifyin gfoifcuitjI16 operates autor'n' 't1cally tdholdlthe tips bf the-Synchronizing pulses 'ip 'ed fth e circuit just :described i uit 18 is rranged to clip the close 'to'jzero b ias level on A.G.Cvtube forn all signal 1 condition'sJf a An example. or circuitry-for, perform n t e. f et en outlined by the fdiagf m Pi e; 1' is'ii ev' l the ing tub 4 isderrio latediu'the der nodulating circuit 17 fcornp ing a erystalfkliode r'ectifier 36, thevideoffre- 'quengy'vvave andf th'e 4.5 mc 'jfreguency sound; wave {obtained at the anode offthecrystal diode rectifier vt t6is anode. or j the" mar ,LF. "arrliplifyapplied 't o the on rqit ri'qref a pentode video frequen'cy amplifying tube 38 pf the YF amplifying circuit The 4.5 rrlc frequency rnodulated sound wz veis derived from the anode =39 and applied by vay o'f a eoupfling capacitor :40, Theibias prov idedbyjresistor' 42 and the detector voltage is "setSO that the 4.5 rnc. ,frequerrcy modulated wave; amplified in theilinear Portion of the Characteristic curve ofthe VJF.wan plifielfituhe 38. As -shown' the. V;F. amplifying circuit l8 is designed for ofthis tube so that the tips ofthe:.synchronizingpulses applifica'tio ni tdbrightnesis and; color diiference cir cuitry v of. a color television receiver translator 'circuitj19 but will .,ser.ve equally syen; as the V13. amplifier of a k-e d-i-white vi h ee .e ig n er-e mn n utera er akee emitheiunetiehp he e t e'dexesi q ing-1a primary windingc4'4 .eopnected --in series Twith a load tained and applied to the'colo'r diffrencebir'iiit'ry of the translator circuit 19. By means of a series resistance element 50 connected to the anode 39 of the V.F. amplifying tube 38 the composite video wave is applied across an input resistor 52 ot an A.G.C. generating tube 54 and to a double time constant circuit comprising a series capacitor 56, parallel connected capacitor .;'57;1and resistor 58 and a shunt resistor 59 to the grid 61 of a synohronizing pulse amplifier tube 62, The capacitor 57 and th'e'r'esis'tor 58;of double t me-co st tutedwork parallel seetion'hre' given values at which" the noise pulse amplitudeyis" reduced and '.the coupling capacitor 56 and shunt resis tor 59 are arranged to provide grid leak bias 'forthe amplifier'tube "62 to clip the synchronizing pulses, both vertical and horizontal, from the composite video wave. 'A second pulse amplifying and pa e het 4; is ree p r to ,1t e. rs y eh ehizi g pu se mpl w a d per qt beiflg e ical sy h zih r s r ve oped .a 'r sse o a e divide e iste fifit and pp i d by wa of jee pl in c paci r r t e djo -a ontr t be 72 e-trequ uey e tre circuitry. I Thegrid of 2a blocking os cillatorftuhe 74 is P d. t0 e entt l e 2-fQ deve Qpi easawe th vqlte e wer t hisz :i applied to t rvidt-et -a hor zo l.fleflection waye ampl fy n ub 1 inthehe imm. sle etiqhmav amp ify ng ei eu t wtoo efle ti a es. .h izen l s a n .f qu n v-are duced int e sec ndar w ndings: of the hor zonta tpu tra o me 8 e d pp d. t .Lthe r z n el deeet eh.eye emq i dihg of we s etiqh 1 ,484; A flyback. tin v ltage. pul o t e zQ filI; f 00] ol s (w t e p et te eun t hpe l n h r ss t eputput ind ng 1. eppliedrhy :m eh tef a eh in vcapaeite 6;to.-t anod ;0 t e A tC-r e 1 h s. vo rpu s gi al enua d. e dud e e t a d :a s ies :ci eu teemp singa capacitor 88: and a resistor 89 .and addedrzto. the e pe v se nwav 1at -t g ofi e o c lla or c n r l e. fi hyrw y. of e. up ng p itor .169 .10 ffect 1 lay twe n the gn nd. e h riz nt l. ynehrez n hulsebx r h pin t wa efo m t9;p ese mor tim'e for-ection OfthQlQOIltfOl tube.l-2.h

The flybaek p ll hpn 'i eenedet t th A A-Q6 ub 54 r si g the a o e netentia t the Qt em r400 elt a s n he A.G C 2t ndue s eeetth ethede s; e nneeted ep i t of; p stt e petent al equa the direct. p tentia g heeempq i eewwe a h enede of the ve -a mp er; tube whi h.: E. th de -e O -w tter eid t el t ee h v.v en e -1-t A.G.C. tube 54 is dependent upon the amplitude ofthe o posite v deo a e; a ie t :t :e l "dur n the gating "interval which charges the. capacitor Q86 in. proportion with the strengthiof the signal. Capacitor 186 is discharged -,dur ing the. idle period of the A.G. C.; tube cycle through the A.G.C. filter network to, apply 5311 A.G.C. voltage of the order of 2 1?; yolts-negatiye. be-

weehl he A G C- u tfla d nt i nt n up n th stre fng tll of thejreceiye d signal ji The AC l. C. voltage; applied through the, seoondaryiwindingj'fil to the grid of the asso 'c ia 'I.- F. arnplifyin'gtube 32.10 vary the gain are relatively close, Within 2-4 volts, to -zerobiasion the tube 54. Wheu the voltage of the composite video, Waye, exc ed; 200 volts, attheig'rid of the tube 5 4 conduction :Will be ,efiected betweenthe, grid and eathode to clip' thoseportions ,of any noise pulses exceedigg the Qvo1r1 hode eve ie-illu trated, in 'F i hout afiecting theoperationi of t the A.G.C. tube; 54 A during the fiybackipulse or Egating; interval: :sinc'e, the anoderof; the A.GzQ; tube 154; is themata greater positive. voltageithan.

fying tube 38 may be operated in a higher g region providing additional gain for both sound and video signals.

Tests of an embodiment of the invention constructed along the lines shown in a schematic diagram of Fig. 2 and using the component values listed below provided an increase of gain of the V.-F. amplifying tube of more than 2 to 1 over the circuit arrangement as wired prior to the application of the circuit according to the invention as well as positive immunity against noise in of the tips of the synchronizing pulses.

Ref. No. Component Type or Value I.-F. amplifier tube 6A Z8 I.-F. amplifier tube- V2 6AN 8 Crystal diode rectifier lNGO.

-F. amplifier tube 6OL6. Coupling capacitor 6.8 mrnf Load resistor k0. Isolating resistor- 68 k0. Grid resistor 150 k0.

A (3.0. tube 6AU6. Coupling capaclto 0.01 ml. Series capacitor" 330 mrnf. Series resistor- 470 k0. Grid resistor 2.7 mego. Pulse amplifying tube- 36 6AN8. Load resistor 2.2 mego.

470 mini. 56 rnmf. 8,200 o.

The power supply used with the circuit arrangement developed 400 volts to the load resistor of the V.-F. amplifier tube 38 and minus 20 volts to the points marked with the minus symbol. The cathode of the A.G.C. tube 54 was maintained at 200 volts and the other points marked with the plus sign were given the usual voltages between 150 and 300 volts. Other component parts values will be determined by those skilled in the art for other applications of the circuitry according to the invention.

The invention claimed is:

1. A television receiving circuit arrangement, including a video frequency amplifying tube having an input circuit and an output circuit, means to apply a composite video signal to said input circuit, means to derive from said output circuit an amplified composite signal with the tips of the synchronizing pulses positive most, an automatic gain control voltage generating tube having at least a cathode, a grid and an anode, a resistance element interconnecting the grid of said generating tube and the output circuit of said amplifying tube, means for maintaining the cathode of said generating tube at a predetermined value of fixed direct potential, means to apply a gating pulse to the anode of said generating tube to render the same conducting in proportion to the amplitude of said amplified composite signal and to produce a gain controlling voltage, means connected between the anode of said generating tube and the input circuit of said amplifying tube to vary the signal level at the output circuit of said amplifying tube in accordance with the amplitude of signal to maintain the gridcathode voltage of said generating tube at substantially zero level in the presence of said synchronizing pulses, whereby any amplitude excursions of said amplified composite signal beyond the tips of the synchronizing pulses on the grid of said generating tube are clipped by diode ing an intermediate frequency amplifying tube, a demodulating device connected to said amplifying tube to derive a composite video signal, a video frequency amplifying tube having an input circuit coupled to said demodulating device and an output circuit from which an amplified composite signal with the tips of the synchronizing pulses positive most is derived, an automatic gain control voltage generating tube having at least a cathode, a grid and an anode, a resistance element interconnecting the grid of said generating tube to the output of said video frequency amplifying tube, means maintaining the cathode of said generating tube at a predetermined value of fixed direct potential, means to apply a gating pulse to the anode of said generating tube to render the same conducting in proportion to the amplitude of said amplified composite signal and to produce a gain controlling voltage, means for applying said gain controlling voltage to said intermediate frequency amplifying tube to vary the gain thereof in accordance with the amplitude of signal whereby any amplitude excursions of said amplified composite signal beyond the tips of the synchronizing pulses on the grid of said generating tube are clipped by diode action of the cathodegrid circuit of said generating tube, a pulse separating tube having an input circuit, and signal coupling means connected between the input circuit of said separating tube and the grid of said generating tube.

3. A television receiving circuit arrangement, including a signal amplifying tube having an input circuit to which a signal having pulse components is applied and an output circuit from which there is derived an amplified signal with the tips of the pulse components positive most, an automatic gain control voltage gene-rating tube having at least a cathode, a grid and an anode, means for connecting the grid of said generating tube to the output of said amplifying tube, means for maintaining the cathode of said generating tube at a predetermined value of fixed direct potential, means to apply a gating pulse to the an ode of said generating tube to render the same conducting in proportion to the amplitude of said amplified signal and to produce again controlling voltage, means for maintaining the grid potential of said generating tube substantially at zero bias level during the application of the pulse components of said signal whereby any amplitude excursions of said signal beyond the tips of the pulse components on the grid of said generating tube are clipped by diode action of the cathode-grid circuit of said generating tube, a synchronizing pulse separating tube having at least a cathode, an anode, and a grid, signal coupling means connecting said last named grid to the grid of said generating tube, whereby the clipping action of said generating tube also serves said separating tube, signal coupling means connecting the cathode of said signal coupling generating tube and the cathode of said synchronizing pulse separating tube to a point of reference potential.

4. A television receiving circuit arrangement, including a video frequency amplifying tubehaving an input circuit to which a composite video signal is applied and an output circuit from which an amplified composite signal is derived with the tips of the synchronizing pulses positive most, an automatic gain control voltage generating tube having at least a cathode, a grid and an anode, a resistance element interconnecting the grid of said generating tube and the output of said amplifying tube, means for maintaining the cathode of said generating tube at a predetermined value of fixed direct potential, means to apply a gating pulse to the anode of said generating tube to render the same conducting in proportion to the amplitude of said amplified composite signal and to produce a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,942,062 June 21, 1960 Albert Macovski It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 68, for "applification" read application Signed and sealed this 6th day of December 1960.,

( SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSUN Attesting Ofl'icer Commissioner of Patents 

